The present invention relates to a heat-sealable connector used for making electrical connection between arrays of electrode terminals on an electronic display device such as liquid crystal display units, electroluminescence display units, light-emitting diodes, electrochromic display units, plasma display units and the like and a circuit board mounting the driving circuit therefor or between electrode terminal arrays on two electric circuit boards as well as to a method for the preparation thereof.
As is known, heat-sealable connectors are widely used heretofore for establishing electrical connection between an electronic display unit such as those mentioned above and a circuit board mounting a driving circuit therefor including rigid and flexible printed circuit boards or between two circuit boards.
Along with the trend in recent years in the technology of electronic display technology toward more and more increased size of the display screens, shift from monochrome displays to colored displays and increased fineness of the picture elements, the number of the electric circuits on various circuit boards to be electrically connected by using heat-sealable connectors is also rapidly increasing. Accordingly, a single heat-sealable connector is required to contain as many as 100 to 700 electroconductive lines which can be achieved only with a decrease in the width of each of the electroconductive lines and pitch of the arrangement thereof. For example, the requirement in this regard is that the width of each of the electroconductive lines is from 0.07 to 0.15 mm and the pitch of the arrangement thereof is from 0.15 to 0.30 mm. While the most conventional method for the formation of an electroconductive line pattern on heat-sealable connectors and the like is the method of screen printing with an electroconductive paste or ink, difficulties are also increasing so much in forming an electroconductive line pattern by the screen printing method.
To explain in more detail, an electroconductive line pattern is formed by the screen printing method by using an electroconductive paste containing fine electroconductive particles dispersed in an organic resinous binder and, in conducting the screen printing on a substrate surface, the layer of an electroconductive paste applied to a fine-mesh printing screen, which is a plain- or diagonalwoven fabric of fine filaments having a diameter of about 10 to 30 .mu.m, is first divided by the filaments or wires and the crossing points thereof to fill the mesh openings into tiny portions which are extruded out of the mesh openings and transferred to the substrate surface where the tiny portions are conjoined together to form a line pattern of the electroconductive lines followed by lifting of the printing screen from the substrate surface.
It is an unavoidable phenomenon in the lifting of the printing screen that a shearing movement is caused between the screen meshes and the layer of the electroconductive paste on the substrate surface so that the consistency of the paste is thixotropically decreased resulting in the phenomenon of so-called running of the paste to increase the width of the patterned lines sometimes by 30 to 50 .mu.m as compared with the width of the openings in the printing screen. Needless to say, this phenomenon is very detrimental when a very fine electroconductive line pattern is desired on the substrate surface due to eventual short-circuiting between adjacent lines. Attempts to solve this problem have been directed mainly toward improvement in the rheological properties of electroconductive pastes by increasing the viscosity or by modifying the thixotropic behavior of the paste and improvement of the formulation of the paste so that the consistency of the paste is rapidly increased immediately after printing on the substrate surface so as to prevent the paste from running on the substrate surface. These measures to modify the rheological properties of the electroconductive paste naturally have limitations relative to the selection of the ingredients of the paste and unavoidable decrease in the printability with the paste resulting in a decreased printing velocity and eventual line break in the printed line pattern. A countermeasure conventionally undertaken is to have a mesh opening of the printing screen having a width smaller by 30 to 50 .mu.m than the desired line width and to conduct printing with an electroconductive paste of a relatively low consistency with good printability so that the printed lines may have a width just as desired even after the unavoidable broadening by the phenomenon of running of the paste after printing.
When the alignment pitch of the electroconductive lines is as small as 0.15 to 0.30 mm, the above mentioned method of using a printing screen of downsized mesh openings is no longer applicable because the width of the mesh openings must be so small with a relatively large proportion of the downsizing resulting in poor penetrability of the electroconductive paste through the printing screen eventually to cause line break of the electroconductive lines on the substrate surface. Thus, this method is under a limitation relative to the alignment pitch of the electroconductive patterned lines.
Moreover, the above mentioned problem relative to the limit in the alignment pitch of the electroconductive lines is greater when, instead of a conventional electroconductive paste compounded with very fine electroconductive particles, the electroconductive lines are formed from a paste compounded with electroconductive particles having a relatively large particle diameter with an object to obtain anisotropic electroconductivity because such large particles can hardly penetrate the fine mesh openings.
As is described above, heat-sealable connectors prepared by the method of screen printing cannot be free from fatal defects when the electroconductive line pattern is so fine in compliance with the requirements in recent electronic devices as to cause a decrease in the penetrability of the electroconductive paste through the printing screen eventually resulting in occurrence of constrictions or break points in the electroconductive lines. When the line width of the electroconductive lines is not so small as to balance with the small pitch of the line alignment, troubles due to direct short-circuiting between adjacent electroconductive lines may eventually be caused as a result of running of the electroconductive paste or, even if this is not the case, troubles are sometimes unavoidable due to displacement of the electroconductive lines caused by inaccurate positioning of the heat-sealable connector in the assemblage of the electronic device or accumulated displacement of the electroconductive line pattern per se also resulting in short-circuiting.
Furthermore, the above mentioned method by using a printing screen of a downsized mesh opening must be accompanied by a troublesome procedure for the adjustment of the consistency of the electroconductive paste and/or modification in setting of the printing conditions. These modifications in the manufacturing conditions of heat-sealable connectors are more or less responsible for various troubles such as an undesirably great increase in the line width of or blurr or running in the electroconductive patterned lines as a consequence of an excessively large volume of the supply of the electroconductive paste due to mismatching between the consistency of the electroconductive paste and the printing conditions not to ensure reliable insulation between adjacent electroconductive lines or, to the contrary, when supply of the electroconductive paste goes short, a complete electroconductive line pattern without line break can hardly be obtained so that no good production efficiency can be obtained by the method of screen printing. Much less, the method of screen printing is not applicable to patterning of electroconductive lines in heat-sealable connectors when the alignment pitch of the electroconductive lines is 0.20 mm or smaller.